Process for the removal and recovery of fluorides from trialkali phosphates



Patented Nov. 5, 1940 1 PATENT OFFICE PROCESS FOR THE REMOVAL AND RECOV- ERY OF PHOSPHATES FLUORIDES FROM TRIALKALI Fredrick C. Jelen, Anniston, Ala, assignor to Monsanto Chemical Company, a corporation of Delaware No Drawing. Application February 23, 1939, Serial No. 257,968

6 Claims. (01. 23407) This invention relates to the removal and recovery of fluorine compounds such as fluorides from alkaline phosphates.

In the manufacture of alkaline phosphates from phosphate rock, the products obtained are in most cases combined or mixed with fluorine compounds particularly fluorides. For the further processing or sale of such phosphate it is, in most cases desirable that the fluorides be either completely removed or at least considerably reduced. It is an object of the present invention to provide a simple process for the purification of alkaline phosphates by removal of associated fluorine. It is a further object of this process to recover such fluorine compounds directly and in a substantially pure form.

My process will be explained by means of the following example:

Example A sample of fluoride trisodium phosphate hydrate containing 20% P205 and 2.45% F as NaF was dissolved by heating in its own water of crystallization and then heated to a temperature of C. Pressure was applied to the solution in order to prevent boiling. The solution was then filtered while at this elevated temperature and thus separated into a residue and a filtrate. The residue after drying contained 37.5% Na3PO4 and 62.5% NaF. The residue may be recrystallized for recovery of pure sodium fluoride. The'solution was crystallized by cooling without the addition of more water and yielded a trisodium phosphate containing 47.3% NasPOi; 0.6% NaF and 52.1% H2O. The process as illustrated by this example therefore enriched the residue from an original molal ratio of NaF to Na3PO4 of 0.46 in the fluoride trisodium phosphate to a molal ratio of NaF to NazPOs in the residue of 6.50.

For the purpose of the present process the concentration of the salts is critical and it is preferred that the amount of water used be maintained within an upper limit of 20 mols of water to 1 mol of NaaPOi and a lower limit of 5 mols of water to 1 mol of Na3PO4.

The efficiency of the separation at 120 is greatest when the ratio of water to NaaPOa. is about 10 or 11 mols of water to 1 mol of NaaPOi. When the molal ratio of water to Na3PO4 is as great as 20 to 1 the process is operable, however the amount of fluoride in the filtrate is greater than with lesser amounts of water.

When the molal ratio of water to Na3PO4 in the solution to be filtered is as low as 5 to 1 the process is still operable, however the fluoride residue is contaminated with Na3PO4.

In general the phenomena which I-have discovered takes place beginning at about 100 C. I as the lower temperature limit. There appears to be no upper temperature limitation upon the phenomena itself and hence the upper temperature limit follows from practical considerations only. The practical considerations are that the water in the concentrations stated, must be in 10 the liquid phase. In order to insure that this is the case, pressure may be applied to the solution to prevent evaporation of water, and hence to maintain the concentrations within the limits stated above. For ordinary purposes tempera- 15 tures between 100 and C. are suitable.

The compositions containing sodium fluoride and trisodium phosphate which may be treated by my herein described process may exist orig- The above double salts may be treated by 30 my herein described process and the sodium fluoride may be separated therefrom in a substantially pure form.

In many cases the trisodium phosphate-sodium fluoride compound may not have been 35 separated from solution as a crystalline compound but the constituents thereof may be present in solution, not only in the proportions given above but in other proportions. Irrespective of whether the trisodium phosphate-sodium fluo- 40 ride composition exists as a true compound or a mixture or merely in solution, the process herein described may be applied thereto and the trisodium phosphate separated from the sodium fluoride.

The process herein described and claimed may be carried out in the presence of small amounts of other salts or compounds. There may for example be present small amounts of caustic soda, NaOH, or sodium carbonate, NazCO3 with- 50 out eifect upon the process.

Having now described my invention and the manner of carrying it out I desire that it not be limited except as indicated by the prior art or as particularly defined by the claims.

What I claims isfi 1. The process for removing sodium fluoride from trisodium phosphate comprising forming a solution containing sodium fluoride and trisodium phosphate adjusting the concentration of said solution to contain between 20 mols of H20 and v5 mols of H20 to 1 mol of NazPO; and then filtering the solution at temperatures above C. to remove crystallized sodium fluoride, NaF, therefrom.

2. The process for separating sodium fluoride and trisodium phosphate comprising adjusting the concentration of a solution containing sodium fluoride and trisodium phosphate so that it will contain between 20 mols and 5 mols of H20 per mol of NasPO4 and then filtering the said solution at temperatures between 100 C. and C. and thereby removing crystallized sodium fluoride, NaF, therefrom.

3. The process for separating sodium fluoride and trisodium phosphate comprising adjusting the concentration of a solution containing both sodium fluoride and trisodium phosphate so that the solution will contain between 20 and 5 mols of H20 per mol of Na3PO4 and then filtering the solution at temperatures between 100 C. and 150 C. while under superatmospheric pressure and thereby removing crystallized sodium fluoride therefrom.

4. The process for preparing trisodium phosphate low in fluoride from trisodium phosphate containing appreciable amounts thereof comprising preparing a solution of said fluoride trisodium phosphate, said solution containing between 20 and 5 mols of H20 per mol of NasPO4, filtering said solution toremove sodium fluoride, NaF while at a temperature above 100 C. and crystallizing trisodium phosphate from the flltrate.

5. The process for separating sodium fluoride from tllSOQillHl phosphate comprising preparing a solution containing said salts so as to have a concentration of about 10 to 11 mols of water to 1 mol of Na3PO4, and then filtering the solution at a temperature above about 100 C.

6. The process for separating sodium fluoride from trisodium phosphate, comprising preparing 20 a solution containing said salts so as to have a concentration in the neighborhood of 10 to 11 mols of water to 1 mol of N a3PO4, and then filtering the solution at a temperature above 100 C.

while said solution is maintained under supe 25 atmospheric pressure.

FREDRICK C. JELEN. 

